Electrochemical cells, usually classified as fuel cells or electrolysis cells, are devices used for generating current from chemical reactions, or inducing a chemical reaction using a flow of current. A fuel cell converts the chemical energy of a fuel (e.g., hydrogen, natural gas, methanol, gasoline, etc.) and an oxidant (air or oxygen) into electricity and waste products of heat and water. A basic fuel cell comprises a negatively charged anode, a positively charged cathode, and an ion-conducting material called an electrolyte.
Different fuel cell technologies utilize different electrolyte materials. A Proton Exchange Membrane (PEM) fuel cell, for example, utilizes a polymeric ion-conducting membrane as the electrolyte. In a hydrogen PEM fuel cell, hydrogen atoms can electrochemically split into electrons and protons (hydrogen ions) at the anode. The electrons flow through the circuit to the cathode and generate electricity, while the protons diffuse through the electrolyte membrane to the cathode. At the cathode, hydrogen protons can react with electrons and oxygen (supplied to the cathode) to produce water and heat.
An electrolysis cell represents a fuel cell operated in reverse. A basic electrolysis cell can function as a hydrogen generator by decomposing water into hydrogen and oxygen gases when an external electric potential is applied. The basic technology of a hydrogen fuel cell or an electrolysis cell can be applied to electrochemical hydrogen manipulation, such as, electrochemical hydrogen compression, purification, or expansion.
An electrochemical hydrogen compressor (EHC), for example, can be used to selectively transfer hydrogen from one side of a cell to another. An EHC can comprise a proton exchange membrane sandwiched between a first electrode (i.e., an anode) and a second electrode (i.e., a cathode). A gas containing hydrogen can contact the first electrode and an electric potential difference can be applied between the first and second electrodes. At the first electrode, the hydrogen molecules can be oxidized and the reaction can produce two electrons and two protons. The two protons are electrochemically driven through the membrane to the second electrode of the cell, where they are rejoined by two rerouted electrons and reduced to form a hydrogen molecule. The reactions taking place at the first electrode and second electrode can be expressed as chemical equations, as shown below.H2→2H++2e−  First electrode oxidation reaction:2H++2e−→H2  Second electrode reduction reaction:H2→H2  Overall electrochemical reaction:
EHCs operating in this manner are sometimes referred to as a hydrogen pumps. When the hydrogen accumulated at the second electrode is restricted to a confined space, the electrochemical cell compresses the hydrogen or raises the pressure. The maximum pressure or flow rate an individual cell is capable of producing can be limited based on the cell design.
To achieve greater compression or higher pressure, multiple cells can be linked in series to form a multi-stage EHC. In a multi-stage EHC the gas flow path, for example, can be configured so the compressed output gas of the first cell can be the input gas of the second cell. Alternatively, single-stage cells can be linked in parallel to increase the throughput capacity (i.e., total gas flow rate) of an EHC. In both a single-stage and multi-stage EHC, the cells can be stacked and each cell can include a cathode, an electrolyte membrane, and an anode. Each cathode/membrane/anode assembly constitutes a “membrane electrode assembly”, or “MEA”, which is typically supported on both sides by bipolar plates. In addition to providing mechanical support, the bipolar plates physically separate individual cells in a stack while electrically connecting them. The bipolar plates also act as current collectors/conductors, and provide passages for the fuel. Typically, bipolar plates are made from metals, for example, stainless steel, titanium, etc., and from non-metallic electrical conductors, for example, graphite.
Electrochemical hydrogen manipulation has emerged as a viable alternative to the mechanical systems traditionally used for hydrogen management. Successful commercialization of hydrogen as an energy carrier and the long-term sustainability of a “hydrogen economy” depends largely on the efficiency and cost-effectiveness of fuel cells, electrolysis cells, and other hydrogen manipulation/management systems (i.e., EHCs). Gaseous hydrogen is a convenient and common form for energy storage, usually by pressurized containment. Advantageously, storing hydrogen at high pressure yields high energy density.
Mechanical compression is a traditional means to achieve compression. However, there are disadvantages to mechanical compression. For example, substantial energy usage, wear and tear on moving parts, excessive noise, bulky equipment, and hydrogen embrittlement. Pressurization by thermal cycling is an alternative to mechanical compression, but like mechanical compression the energy usage is substantial. In contrast, electrochemical compression is quiet, scalable, modular, and can achieve high energy efficiency.
One challenge for electrochemical hydrogen compression is the safety concern regarding pressurized hydrogen gas. Hydrogen gas is extremely flammable and high pressure hydrogen gas raises safety issues. A major concern can include the leaking or unintended release of the high pressure gas from the electrochemical compressor. A catastrophic release could pose a safety hazard.
Moreover, even a small leak that may not rise to the level of a significant safety concern nonetheless reduces the efficiency of the electrochemical compressor. Therefore, there is a need to prevent or reduce hydrogen leakage.